1. Field of the Invention
The present invention relates to a spectroscope for extracting a desired wavelength component included in measured light by launching the measured light into a diffraction grating so as to perform spectroscopic measurement.
Priority is claimed on Japanese Patent Application No. 2008-015031, filed Jan. 25, 2008, the contents of which are incorporated herein by reference.
2. Description of the Related Art
In a known spectroscope for obtaining a high resolution or a wide dynamic range in spectroscopic characteristics, a multipath system is employed in which measured light is launched into a diffraction grating a plurality of times, and also transmitted through a plurality of slits. As a spectroscope having a multipath system, Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. H08-159877) discloses a Littrow double-path monochromator.
As shown in Patent Document 1, the disclosed double-path monochromator has a diffraction grating, a collimator, and two slits. The measured light emitted from a light-emitting device such as an optical fiber is collimated by the collimator, and is launched into the diffraction grating. The diffracted light is converged by the same collimator, and is transmitted through the first slit. The diffracted light which passed through the first slit is collimated again by the collimator, so as to direct the light into the diffraction grating. The diffracted light is converged again by the same collimator, and is transmitted through the second slit.
That is, the double-path monochromator disclosed in Patent Document 1 has a first spectroscopic path from the light-emitting device to the first slit and a second spectroscopic path from the first slit to the second slit, wherein the measured light is transmitted through the first spectroscopic path and the second spectroscopic path in turn.
In the above-described double-path monochromator, the measured light is spectroscopically dispersed via the first spectroscopic path, and is further spectroscopically dispersed via the second spectroscopic path. Therefore, it is possible to obtain effects similar to those obtained when two optical filters are serially arranged on the optical path of the measured light, and thus to obtain a high resolution and a wide dynamic range.
In the above structure, the first and second slits are separate parts, and cannot be disposed at the same (spatial) position. Therefore, in the double-path monochromator, the incident angle of the measured light on the diffraction grating in the first spectroscopic path is not equal to that of the measured light in the second spectroscopic path, so that the emission angle of the diffracted light in the first spectroscopic path is not equal to that of the diffracted light in the second spectroscopic path, thereby separating the optical path of the diffracted light in the first spectroscopic path from that of the diffracted light in the second spectroscopic path. Accordingly, it is possible to transmit the measured light through both the first and second slits, which are separately arranged.
However, when the measured light is launched into a diffraction grating, not only diffracted light but also scattered light, which is emitted in all directions, are produced. In addition, the known double-path monochromator uses a single collimator in both the first and second spectroscopic paths, so that the measured light emitted from the collimator on the first spectroscopic path and measured light emitted from the collimator on the second spectroscopic path irradiate the same area of the diffraction grating.
Therefore, part of the scattered light, which is produced when the measured light is launched into the diffraction grating on the first spectroscopic path, is emitted in the same direction as the emission direction of the diffracted light on the second spectroscopic path, and thus is superimposed on the diffracted light on the second spectroscopic path. Accordingly, the superimposed light is transmitted through the second slit.
In the double-path monochromator, the scattered light in the superimposed light (together with the diffracted light on the second spectroscopic path) functions as inevitable stray light which degrades the resolution and dynamic range in the spectroscopic characteristics.
Such a problem relates, not only to any double-path monochromator in which the measured light passes through two slits, but also to any multipath spectroscope in which measured light passes through a plurality of slits. That is, when part of the scattered light, which is produced on a spectroscopic path prior to the last spectroscopic path, passes along the same optical path as that of the diffracted light on the last spectroscopic path, the resolution and dynamic range in the spectroscopic characteristics degrade.